10(5 THE VOYAGE OF H.M.S. CHALLENGER. 



serrate margin is divided into three portions defined by a tooth that is larger and more 

 powerful than the rest, each portion looking in a different direction, as shown in the 

 figure (PI. XVIII. d). In many Crustacea, perhaps in most, a massive tuberculated 

 ridge or molar process traverses the base from the posterior to the anterior margin. In 

 this family this process is smooth, and consists of a slender ridge, with the anterior 

 extremity of which a two-jointed synaphipod articulates, the second joint of which is 

 fringed with long hairs and generally lies folded in the hollow formed by the concave 

 psalisiform blades, where it evidently acts as a brush to arrange the food in position during 

 the process of manducation. The whole of this important structure is attached to a long 

 apophysis or lever that is moved on its longitudinal axis by a muscle attached to a tendon 

 just below the base of the psalisiform plate, and diagonally b}^ muscles at the extremity 

 of the apophysis, which are attached to the antero-lateral extremity of the carapace. 



There is very little variation in the form of the mandibles in the various species or 

 genera, and what little there is exists in the serrate margin, which may be more or less 

 coarsely or finely marked, so that the number of denticles in the central division may 

 vary in number. 



The first pair of siagnopoda or maxillae (PI. XVIII. e) consists of two small, 



longitudinally curved plates, as shown in the accompanying cut, that 



lie laterally outside the siagnos, but are pressed firmly against its 



posterior surface. Both the plates are flat, and the outer one is the 



larger, and terminates in one or two strong spines. At the base of the 



outer plate is a compact tuft of ciliated hairs. This fasciculus likewise 



fig. 19.— First maxilla, x 3, exists attached to the first pair of siagnopoda in the Scyllaridse, to 



From^a drawing "by which the whole appendage bears a greater resemblance than to that of 



any other of the Macrura; it is very unlike the same in the Astaeidae. 



There is very little variation of this part in the various species, and what differences 



there may be appear to lie in a tendency for the outer branch to change in its relative 



proportion to the size of the inner. 



The second pair of siagnopoda or maxilla? (PI. XVIII. f) consists 

 of two small plates, almost rudimentary in character, and a large 

 squarnose plate that is projected forwards as far as the anterior extremity 

 of the mouth. The margin of the plate is fringed with short cilia, all 

 directed towards the anterior extremity. The two small julates that 

 are the rudiments of the normally formed appendage are tipped with 

 lone; hairs, and are folded back against the great squarnose plate. The 



Flu. 20.— Second maxilla, ° S> o 1 1 



b wiikluoes a suhm Viug seconc ^ i s implanted outside, and but little behind the first pair, and 



forms an efficient valve or doorway capable of shutting up the exit 



passage from the branchial chamber, so that while the animal might safely burrow in the 



mud, pure w T ater could be retained in the branchial chamber, and irritating detrital matter 



